In holographic information storage, an entire page of information is stored at once as an interference pattern within a thick, photosensitive material comprising a holographic data storage layer. This is done by intersecting two coherent laser beams within the storage layer. The first, called the data beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce, for example a simple collimated beam with a planar wavefront.
The resulting interference pattern causes chemical and/or physical changes in the photosensitive medium: a replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium.
When the stored interference pattern is illuminated with one of the two waves that were used during recording, some of this incident light is diffracted by the stored interference pattern in such a fashion that the other wave is reconstructed. Illuminating the stored interference pattern with the reference wave reconstructs the data beam, and vice versa.
Optical drives, including CD and DVD drives, detect variations in the optical properties in the surface of an optical data storage layer. Such optical drives direct a light beam onto that surface and detect either the presence or absence of a corresponding reflected beam.